1. Field of the Invention
The present invention relates to a scheduling apparatus of a base station supporting an Automatic Repeat reQuest (ARQ) method. In particular, disclosed is a scheduling apparatus for scheduling a data unit transmitted from an MAC that is a base station providing the ARQ method for correcting an error in wireless data communication and generating a high rate frame. The present invention is derived from work as a part of a new driving force of core IT technology development work performed in at the Ministry of Information and Communication and The Institute for Information Technology Advancement [Ref. No. 2006-S-001-02, title: adaptive wireless access and transmission technique for 4G mobile communication system].
2. Description of Related Art
Research is vigorously performed to provide a variety of Quality of Service (QoS) having more than 100 Mbps transmission rate in the 4th generation communication system that is a next-generation communication system. In order to accomplish this, a mobile communication system evolves into a form of Broadband Wireless Access (BWA) such as a Local Area Network (LAN) system and a Metropolitan Area Network (MAN) system, plus guaranteeing mobility and QoS.
An error may occur in specific data in wireless data communication according to wireless channel status.
Methods for controlling an error occurring from loss or damage of a frame during data transmission in wireless environment are a stop-and-wait ARQ method, go-back-N-continuous ARQ method, selective-repeat continuous ARQ method, and the like. Further, a hybrid ARQ method is recently used.
According to the stop-and-wait ARQ method, a transmitter waits for an Acknowledgement (ACK) message after transmitting a single frame, a receiver transmits an ACK message only upon successively receiving a frame and transmits a Negative Acknowledgement (NACK) message when error occurs in the frame. When a receiver fails to receive a frame due to destruction of the frame, the receiver does not transmit any signal and a transmitter retransmits when a signal is not received within a predetermined time. Thus, the stop-and-wait ARQ method has an advantage that it is easily realized but has a disadvantage that transmission efficiency declines as waiting time increases.
Also, according to the go-back-N-continuous ARQ method, a transmitter transmits a series of frames with a fixed window-size and a receiver does not receive the series of frames when an error is detected from at least one frame in the series of frames and transmit NACK message. The transmitter retransmits all frames after the NACK message.
Also, according to the selective-repeat continuous ARQ method modified from the go-back-N-continuous ARQ method that retransmits only the frame that receives the NACK, it has good transmission efficiency, but overhead may occur from a memory space for storing a next frame until a frame where an error occurs is retransmitted. Specifically, according to the selective-repeat continuous ARQ, error checking result (e.g., CRC result) for packets received in a receiver should be feedback to a transmitter. First of all, a transmitter performs initial transmission of a packet and a receiver decodes a received packet. In this case, when an error does not occurs, ACK signal is transmitted to the transmitter. Conversely, when an error occurs, NACK signal is transmitted to the transmitter. The transmitter either retransmits a previous packet or transmits a new packet according to ACK/NACK signal from the receiver.
Generally, scheduling is performed in consideration of channel environment of a mobile station and priority of service. However, when a retransmission packet exists, the retransmission packet must be scheduled to be sent firstly among the packet having the same priority. When retransmission of a retransmission packet is delayed, a timeout may occur and other packets related to the corresponding packet may be discarded. When initial transmission is delayed, it appears as service delay, but when retransmission is delayed, it causes the discard of a packet and appears as service cut off. However, when initial transmission having high-priority competes with retransmission having low-priority under a condition that priority of each service is completely different, a system may fall into a dilemma for determining that one should be firstly transmitted. If scheduling is performed according to priority and if data having high-priority are in a rush in a specific time period, a packet having low-priority may not be retransmitted continuously, so that other packets related to the retransmission packet may be at risk of being discard.